1. Field of the Invention
The invention relates generally to sensor nodes and methods for operating distributed collections of sensor nodes.
2. Discussion of the Related Art
This section introduces various related aspects of the art, which may facilitate a better understanding of aspects of the present invention. While the statements are explanatory, they should not be understood to be admissions of prior art.
It is often desirable to know the spatial locations of the nodes of a network. The spatial locations may, e.g., be determined by directly measuring the distances of the nodes from fixed spatial markers. Such distance measurements may be made physically, e.g., with tape measures, or may be made indirectly with satellite global positioning system (GPS). In either method, the determination of the spatial locations each node of the network may be costly, because the measurements involve either costly human intervention or costly equipment, e.g., multiple GPS transponders.
To reduce expenses, alternate methods have been proposed for determining the spatial locations of the nodes of networks. The alternate methods are almost coordinate-free, because they rely primarily on determining relative locations of node pairs. From the relative locations, absolute spatial locations of the nodes may be determined by combining the measured relative locations with the measurement of the absolute locations of one or a few nodes. For example, determination of absolute locations of the nodes of a network may involve using GPS transponders to determine the absolute spatial locations of one or a few nodes rather than using a GPS transponder to determine the absolute spatial location of each node of the network.
One such alternate method for spatially locating the nodes of a network uses direct communications between node-pairs. According to this method, pairs of nodes communicate directly with each other, i.e., without retransmission via third nodes. Herein, these direct inter-node communications are referred to as “inter-node chatter”. During inter-node chatter, one node of a pair transmits, e.g., a signal having a known strength or a known transmission time, and the other node of the pair measures, e.g., the strength of the signal or the arrival time of the signal. From such strength and/or arrival time measurements, the receiving node estimates the attenuation of the signal or the transmission delay of the signal. By comparing the attenuation or transmission delay for inter-node chatter from different transmitting nodes, a receiving node estimates the relative distances of said transmitting nodes, e.g., to determine which transmitting node or nodes are closest.
While this alternate method may enable determinations of relative spatial locations of nodes, methods based on inter-node chatter are often undesirable. Indeed, some types of sensor nodes do not exchange signals with each other and are commonly known as anchorite sensors. In large sensor arrays, e.g., arrays having 105-107 sensor nodes, anchorite sensors can provide important advantages. In particular, in large sensor arrays, equipping individual nodes to support inter-node chatter would be significantly more costly than making the nodes anchoritic. Thus, the factor of cost may make inter-node chatter an undesirable tool for use in determining the spatial locations of individual nodes.